Effect of the Nitrogen Ions Implantation Mode on the Structure-Phase Transformations in Graphite Cast Irons

Author Name(s): Evgenij R. Akst, Lenar N. Shafigullin, Irina F. Shaekhova
Author Email: ev.akst@yandex.ru

Abstract

Using nuclear gamma resonance (Mössbauer) spectroscopy in the backscattering geometry, the surface layers of graphite cast irons irradiated with nitrogen ion fluxes were studied. New phases were discovered and identified that are formed in the surface layers of the studied alloys during the implantation of nitrogen ions. A relationship has been established between the type of new phases, their relative amount and the implantation (irradiation) regime of cast irons. The processes accompanying ion implantation are considered, and the mechanisms of formation of secondary phases in cast irons, taking into account the presence of graphite inclusions in their structure, are proposed. The effect of the density of the ion current (implantation current) on the formation of new phases is studied.

Introduction

Ion implantation is one of the effective ways to control the properties of materials. In recent decades, the possibilities of ion implantation for modifying the surface properties of metals and alloys have been actively studied, which led to the formation of a new direction in ion-beam technology – implantation metallurgy [1]. Of particular interest is the prospect of modification through ion implantation, in particular, the implantation of nitrogen ions, the surface properties of graphite cast irons. Graphite cast iron is a relatively inexpensive, technologically advanced and widespread structural material. At the same time, the complex chemical composition and the presence of graphite inclusions in the structure force graphite cast irons to be classified as rather complex, inhomogeneous metal systems in which radiation-induced processes can proceed in a special way [2,3]. Currently, the use of nitrogen ion implantation to modify the surface of graphite cast irons is constrained by the lack of scientifically sound recommendations on the choice of irradiation (implantation) regimes, based on the understanding and consideration of all processes that accompany ion implantation. This, in particular, concerns the processes of the formation of new phases in the implanted layer.

Conclusion

The comparison shows that at an ion current density of 5 μA / cm2 in the surface layers of cast iron there is a greater number of new phases than at j = 50 μA / cm2. Moreover, under conditions of lower ion current density, mainly finely dispersed ε-carbides of the Fe-C-Si type are formed, as well as paramagnetic ε-nitrides (carbonitrides), which are close in composition to Fe2N. With an increase by an order of magnitude of the ion current density in the implanted layer, the fraction of γ’-nitrides increases, and ε-carbides turn out to be in the ferromagnetic state. In addition, the composition of ε-nitrides (carbonitrides) formed in this case approaches Fe3N, which makes them also ferromagnetic. These differences can be explained by a change in the dominant processes in the irradiated material. Probably, at an ion current density of 5 μA / cm2 , radiation-induced segregation and ionic mixing, which determine the dynamics of phase formation, predominate in the surface layers of cast iron. The process of radiation-induced segregation caused by the movement of point radiation defects to sinks leads to the accumulation of nitrogen and silicon atoms at the boundaries of graphite inclusions in cast iron, which increases the likelihood of nucleation of ε-nitrides enriched with nitrogen, as well as finely dispersed ε-carbides of the type Fe-C- Si.

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